The present invention relates to a method for fabricating a semiconductor device. More particularly, the present invention relates to a method for fabricating a semiconductor device including the step of turning a polycrystalline or amorphous semiconductor layer formed on an insulator body into a single crystal layer.
As a semiconductor device in which a semiconductor layer is formed on an insulator body, an integrated circuit (IC) is known wherein a silicon dioxide film is formed on the surface of a single crystal silicon substrate, a silicon layer is formed thereover by the CVD process, and the resultant silicon layer is utilized as a gate electrode or a wiring layer. A semiconductor device of SOS structure is also known wherein a silicon layer is epitaxially grown on a sapphire substrate and elements are formed in this epitaxially grown silicon layer. However, the silicon layer formed on the silicon dioxide film by the CVD process or the like does not become a single crystal layer but becomes a polycrystalline silicon layer having a relatively high resistance. For this reason, a semiconductor device utilizing this silicon layer as the gate electrode or as wiring has been defective in that the operating speed is reduced. Furthermore, with an SOS type semiconductor device as described above, when the surface of the sapphire substrate used has an incomplete crystal plane, the silicon layer epitaxially grown thereover will have corresponding crystal defects. Therefore, only sapphire substrates of high quality can be used.
A method has been proposed to overcome these problems by converting the polycrystalline silicon layer deposited on the silicon dioxide film or the like into a single crystal layer. An example of this method is known as "graphoepitaxy", to be described below. According to the graphoepitaxy method, a number of grooves having side walls at right angles to the film surface are formed on the entire surface of the insulating film such as a silicon dioxide film. A polycrystalline silicon layer is deposited on this insulating film. This polycrystalline silicon layer is irradiated with an energy beam such as a laser beam to be melted and resolidified in a short period of time. When the polycrystalline silicon layer deposited on the silicon dioxide film having a flat surface is irradiated with the energy beam to be melted and resolidified, it is known that the resolidified silicon tends to grow with its crystal plane (100) oriented in the direction perpendicular to the surface of the silicon dioxide film. In this case, crystal growth in the direction parallel to the surface of the silicon dioxide film is not facilitated. Therefore, only a polycrystalline silicon layer consisting of many crystal grains having a crystal plane (100) in the direction perpendicular to the surface of the silicon dioxide film can be formed. In order to solve this problem, according to the graphoepitaxy method, crystal growth of the resolidified silicon in the horizontal direction (in the direction parallel to the surface of the silicon dioxide film) is facilitated by many grooves formed on the surface of the silicon dioxide film, whereby a single crystal layer of uniform orientation may be formed. In this case, the crystal grows such that the crystal plane (010) is oriented along the longitudinal direction of the grooves and the crystal plane (001) is oriented in the direction perpendicular to the longitudinal direction of the grooves.
Apart from the effects obtained as described above, the surface of the single crystal silicon layer formed by the graphoepitaxy method as described above has a three-dimensional pattern corresponding to the grooves on the surface of the silicon dioxide film. This presents big problems in the patterning of this silicon layer to form electrodes and wiring or in forming semiconductor elements in this silicon layer. The presence of such a three-dimensional pattern entails nonuniformity in the pattern size in the process of photolithography, due to differences in the focal distances. Reflection from the end faces of the three-dimensional pattern also significantly degrades the precision in photolithography.